1. Field of the Invention
The present invention relates to a method for controlling a static convertor for power transmission by means of high-voltage direct current (HVDC), in which the direct current of the convertor is measured and compared with a current order. The convertor is controlled with a control angle which is dependent, on the one hand, on a component proportional to any deviation arising between the direct current and the current order and, on the other hand, on a component corresponding to the time integral of any such deviation. In inverter operation and at a direct current which is smaller than the current order, the convertor is controlled so that its current-voltage characteristic has a positive gradient.
The invention also relates to a convertor provided with a control means for carrying out this method.
2. Description of the Prior Art
Convertors of the above-mentioned kind are previously known from the Specification of British Pat. No. 1,300,226. This specification describes an HVDC transmission system in which the characteristic of the inverter is modified within the region of the current margin by making the extinction angle (.gamma.) current-dependent and increasing it beyond its minimum value (.gamma..sub.O). The characteristic within the current margin region is given a positive gradient and in this way static stability for certain operating cases is achieved. It has been shown, however, that this solution can give rise to instabilities in the operation of the convertor and this is particularly true when the convertor is connected to a weak alternating-voltage network. Despite the static stability provided, natural oscillations may occur in the control system, which result in a relatively large AC component being superimposed on the direct current of the plant. In certain cases it has been possible to remedy these phenomena by making the alternating-voltage networks stiffer with the aid of synchronous condensers or static equipment for reactive power compensation, but such solutions are very expensive.
Another convertor of the initially described kind is previously known from Erich Uhlmann: Power Transmission by Direct Current (Springer-Verlag, 1975), pages 132-133, FIG. 9.6. In this document a method of increasing the stability is proposed which involves making the inverter, within the current margin region, operate with a constant control angle (.gamma.), which gives the characteristic a slightly positive gradient within the current margin region. The attenuation of any oscillations which may arise is, however, weak were the Uhlmann method to be employed.